1. Field of the Invention
The present invention relates generally to a portable device for measuring blood pressure and a method therefor. More particularly, the present invention relates to a portable device for measuring wrist or finger arterial pressure corresponding to brachial arterial pressure and a method therefor.
2. Description of the Related Art
Blood pressure is one of the most useful vital signs for human health care. Clinically, blood pressure provides an index by which abnormalities of the circulatory system, including the heart and blood vessels, can be diagnosed. Blood pressure outside a normal range requires consistent care. Arterial pressure changes with cardiac pulsation. The peak pressure the arteries feel when the ventricles are contracting and pushing the blood out into the arteries is called systolic pressure. Even when the ventricles are relaxing and filling back up with blood, the elasticity of the walls of the arteries maintains some blood pressure, and thus the blood pressure does not drop to zero. This lower level of blood pressure is called diastolic pressure.
Nervousness often results in a high blood pressure measurement when a person visits a clinic. Since blood pressure varies with a variety of factors, it is difficult to get an accurate blood pressure measurement at one time. Hence, a home-use electronic blood pressure monitor is necessary to enable users to constantly measure their blood pressures at their homes. The most widely used automatic electronic blood pressure monitor operates by volume oscillometry. The volume oscillometry scheme measures blood pressure based on oscillations that are generated by raising or dropping pressure to the arteries with use of a cuff. In this measuring method, the cuff pressure at which the pulse arising from cardiac pulsation has a maximum amplitude that is estimated to be a mean blood pressure. Herein, the systolic and diastolic pressures are estimated to be cuff pressures having 45 to 57% and 74 to 82% of the maximum amplitude, respectively.
FIG. 1 is a graph illustrating blood pressure measurements based on oscillation pressures of a cuff. Referring to FIG. 1, the ratio of a systolic amplitude to a maximum amplitude and the ratio of a diastolic amplitude to the maximum amplitude are called characteristic ratios, which vary with individuals. It is known that the characteristic ratios are greatly affected by cuff characteristics, the characteristics of artery vessels, the shape and amplitude of arterial pressure waves, etc.
At present, most blood pressure monitors measure blood pressure in an upper arm at heart level. For measurement convenience, products for measuring blood pressure in a wrist or a finger using a cuff have been developed and commercialized.
Such a wrist- or finger-type blood pressure monitor is smaller than the upper arm-type monitor. Due to the small size, the wrist- or finger-type blood pressure monitor is portable and obviates the need to take off a portion of a patient's clothing for pressure measuring, thus allowing blood pressure measuring at any time. However, because a wrist or finger arterial signal is weaker than a brachial arterial signal, it basically has a low Signal-to-Noise Ratio (SNR). Therefore, the wrist- or finger-type blood pressure monitor is less accurate than the upper arm-type one.
Blood pressure may differ at different artery positions due to a plurality of factors. Hence, the wrist- or finger-type blood pressure monitor may give a different blood pressure measurement from that of the upper arm-type one. As a consequence, blood pressure measurements of the wrist- or finger-type blood pressure monitor may be less reliable to users.
A conventional wrist- or finger-type blood pressure monitor measures an arterial pressure by oscillometry, like the upper arm-type blood pressure monitor. If the blood pressure monitor is precise and the brachial arterial pressure is equal to the wrist- or finger arterial pressure, a pressure at the upper arm should be equal to a pressure at the wrist or finger which is leveled at the heart height.
However, the brachial arterial pressure is different from the wrist or finger arterial pressure due to the characteristics of blood, flow, and the differences between individuals.
That is, a measured wrist or finger arterial pressure of a user may be higher or lower than a measured brachial arterial pressure for the same individual.
FIG. 2 illustrates characteristics of blood pressure and waveform changes with respect to the positions of arteries. In FIG. 2, pulse pressure changes with respect to the distances of the arteries to an aorta are shown, as well as pulse pressure changes that vary with age. That is, the difference between systolic and diastolic pressures increases at an artery more remote from an aorta at younger ages due to the overlap of reflective waves. At older ages, the difference is zero or may increase. Even individuals of the same age may differ in the systolic-diastolic difference. Therefore, when a user familiar with the upper arm-type blood pressure monitor popular in clinics uses the wrist- or finger-type blood pressure monitor, for use convenience or portability, he may be confused with blood pressure measurements.